igneous

Diorite (River Cobble)

Phaneritic Intermediate Plutonic Rock

Hardness: 6-7 (Mohs scale); Color: Salt-and-pepper (mottled white/gray and black); Luster: Dull to vitreous; Crystal Structure: Phaneritic (coarse-grained); Cleavage: Two directions at nearly 90 degrees (plagioclase); Specific Gravity: 2.8 - 3.0

Hardness: 6-7 (Mohs scale)
Color: Salt-and-pepper (mottled white/gray and black)
Luster: Dull to vitreous

Identified April 6, 2026More igneous →

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Physical properties

Formation & geological history

Formed from the slow cooling of intermediate magma beneath the Earth's surface. Often found in volcanic arcs or orogenic belts where subduction occurs. This specimen is a cobble, meaning its rounded shape was formed by water erosion in a stream or riverbed.

Uses & applications

Used primarily in building construction as crushed stone or 'black granite' for countertops, monuments, and decorative facing stone. Smaller rounded cobbles are common in landscaping.

Geological facts

Diorite was used by ancient civilizations for sculpture and inscriptions because of its extreme hardness; the famous Code of Hammurabi was carved onto a Diorite stele.

Field identification & locations

Identify in the field by its distinctive 'salt-and-pepper' look with roughly equal parts dark and light minerals. Unlike granite, it contains little to no visible quartz. Commonly found in mountain ranges or riverbeds near igneous outcrops.

Other igneous specimens

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Blue Apatite

Fluorapatite (Ca5(PO4)3F)

mineral

Dark Igneous Rock (Likely Basalt or Diabase)

Basalt (extrusive igneous rock) or Diabase (intrusive igneous rock - also known as Dolerite), largely composed of plagioclase feldspar, pyroxene (augite), and sometimes olivine and amphibole. Exact mineralogy would require thin section analysis.

Igneous

Magnetite

Iron(II,III) oxide (Fe3O4)

mineral

Blue Apatite

Apatite (Group), Ca5(PO4)3(F,Cl,OH)

mineral

Sodalite

Sodalite - Na8(Al6Si6O24)Cl2

mineral

Magnetite

Magnetite (Fe3O4)

mineral